1. Field of the Invention
The invention relates to high molecular weight aromatic polyamides and a process for preparing them.
2. Discussion of the Background
Aromatic polyamides with the repeating unit EQU --CO--Ar--CO--NH--Ar'--X--Ar"--Y--Ar"--X--Ar'--NH--
are distinguished not only by high temperature resistance and good mechanical properties; they can also be processed thermoplastically (Elias/Vohwinkel, "New polymeric materials for industrial use", 2nd Edition, Carl Hansor Verlag 1983, pages 242 ff.). In this formula, X stands for an ether oxygen and Y for a carbonyl group, and Ar, Ar', and Ar", independently of one another, stand for the p- and m-phenylene groups.
Aromatic polyamides can also be prepared by the reaction of aromatic dicarboxylic acids with aromatic diisocyanates (DE-OS 19 28 435) and by reacting diaryl esters of aromatic dicarboxylic acids with aromatic diamines.
Processes are known in which aromatic polyamides are obtained directly by the reaction of aromatic dicarboxylic acids and aromatic diamines in the presence of aromatic phosphites. Solvents that have proved useful for this process are N-methylamides, especially N-methylpyrrolidone. On the other hand, no polymeric amides are obtained with other dipolar aprotic solvents such as dimethyl sulfoxide (see F. Higashi et al., J. Polym., Sci., Polym. Chem. Ed 18, 1711 ff. (1980)).
A summary provided in S. M. Aharoni et al., J. Polym. Sci., Polym. Chem. Ed. 22, 2579 (1984) teaches that (1) the phosphite used has to contain aryl groups and is preferably triphenyl phosphite; (2) the aryl phosphites have to be used in at least an amount such that for every 1 mole of amide to be converted there is 1 mole of a compound with the group ##STR2##
since this group is consumed during the reaction and this reaction represents the driving force of the conversion; and (3) pyridine is not necessary for the reaction, but at the same time it has an accelerating effect on the course of the reaction.
Besides these processes in which the condensation is carried out in a solvent, there have also been attempts to prepare polyamides in the melt. Thus, U.S. Pat. No. 3,109,836 describes a process for preparing polyamides with the repeating unit (CO--Ar--NH) that consists of heating acetamidobenzoic acid for 3 hours under vacuum at 200.degree. to 300.degree. C. Contrary to the claims in this document, this process does not provide thermoplastically processable products, since the melting points of the reaction products are in the range of the temperature of decomposition or above.
It has also been proposed to prepare aromatic polyamides by transamidation of acylated aromatic amines in the melt. Aside from the fact that such a procedure would require the prior preparation of the acylated amines, the results obtained by this procedure are still considered to be very unsatisfactory. To improve the processability, pure aromatic starting materials were not used, but mixtures with aliphatic compounds. The diamines are not completely but only partially acylated. Finally, acetic acid, acetic anhydride, dimethylacetamide or another agent is added to the reaction melt to improve the flowability. See Keske et al., Polymer Prepr., 25, Part XXV, p. 25 (1984) and U.S. Pat. No. 3,654,227.
As pointed out in the monograph by Buhler "Special Plastics", Akademieverlag, Berlin (1978), on page 412, the method of melt polycondensation cannot be applied to the preparation of aromatic polyamides from aromatic dicarboxylic acids and simple aromatic diamines.